feat(snake): add adaptive adversarial search

- Share duel search contexts and transpositions across candidate moves.
- Add aspiration windows, principal variation ordering, and body caches.
- Model simultaneous multiplayer responses with a compact beam rollout.
- Adapt search depth and response breadth to the remaining deadline.
- Add a deterministic arena benchmark with optional JSON reporting.
- Expose search metrics, document benchmarking, and bump Prism to 1.2.0.

justfile

@@ -62,6 +62,14 @@ bench-best-snake iterations="1000":
 
   PYTHONPATH="{{justfile_directory()}}" python "{{justfile_directory()}}/tests/bench_best_battle_snake.py" --iterations "{{iterations}}"
 
+bench-snake-arena positions="100" output="":
+  #!/usr/bin/env bash
+  set -euo pipefail
+
+  args=(--positions "{{positions}}")
+  if [ -n "{{output}}" ]; then args+=(--json-output "{{output}}"); fi
+  PYTHONPATH="{{justfile_directory()}}" python "{{justfile_directory()}}/scripts/benchmark_snake_arena.py" "${args[@]}"
+
 build-battlesnake-cli:
   #!/usr/bin/env bash
   set -euo pipefail